1. Field of the Invention
The present invention relates to an electric power steering apparatus which is adapted to transmit a driving force generated as a steering assist force by an electric motor to a steering mechanism.
2. Description of Related Art
Electric power steering apparatuses are conventionally used, which are adapted to transmit a driving force generated by an electric motor to a steering mechanism via a gear mechanism (reduction gear mechanism) or by a direct drive system.
In such an electric power steering apparatus, an assist characteristic which defines a relationship between a steering torque applied to a steering wheel and a target value of an assist torque to be applied to the steering mechanism from the electric motor is preliminarily determined, and stored as an assist map in a memory. An assist torque target value for a given steering torque is read out of the assist map. The electric motor is controlled to be driven on the basis of the assist torque target value thus read out.
As shown in FIG. 9, the assist characteristic is defined so that the assist torque target value is increased as the steering torque increases. For example, the steering torque is assigned a positive value for rightward steering, and assigned a negative value for leftward steering. The assist characteristic is defined so that a positive assist torque target value corresponds to a positive steering torque value and a negative assist torque target value corresponds to a negative steering torque value.
When the assist torque target value is positive, the steering assist force acts on the steering mechanism for turning steerable wheels rightward. When the assist torque target value is negative, on the other hand, the steering assist force acts on the steering mechanism for turning the steerable wheels leftward. Where the steering torque has a value within a dead zone around zero, the assist torque target value is set at zero.
The electric power steering apparatus employing the assist characteristic is problematic in that, when a driver performs a return stroke steering operation to turn the steering wheel back to a steering angle midpoint, the driver is liable to have a heavier steering feeling (so-called “spring feeling”) such that the steering wheel is returned to the steering angle midpoint more heavily than intended by the driver. That is, the steering torque is reduced in the return stroke steering operation, so that the assist force is correspondingly reduced. Therefore, the steering wheel receives a reverse input from the wheels thereby to be heavily returned to the steering angle midpoint.
Further, the electric power steering apparatus is problematic in that, when the driver performs a steering holding operation to hold the steering wheel at a certain steering angle, a steering burden is increased.
These problems can be solved by increasing the inclination of an assist characteristic curve so that the assist torque target value is set greater for a given steering torque. In this case, however, a steering response is reduced when the driver performs a forward stroke steering to actively turn the steering wheel.